1. Field of the Invention
This invention relates to the field of plastic foams. More specifically, it relates to a process and apparatus for introducing a gas into at least one of the reaction components so as to obtain a foam with a uniformly sized and distributed cell structure.
2. Description of the Prior Art
When producing molded parts of polyurethane foam, the charge of gas in the initial components has a significant effect on the foam structure and quality of the molded part. Dissolved gases and trapped gas bubbles change the viscosity and density of the component mixture. Gas charging not only affects mixing quality and machine adjustment, it also significantly alters foam structure and foam density distribution. It serves as a nucleating aid.
Even visual examination of the foam parts permits conclusions to be drawn relative to gas charging. With low gas charging, a dark molded part with a few large cells is obtained. As gas charging increases, the foam becomes lighter and the number of cells per unit volume increases, their radius becomes smaller. A qualitatively good foam must contain at least a given amount of gas. This is true both for flexible foams and for rigid and integral-skin foams.
In order to achieve this end, it has already been proposed that gas be introduced directly into the mixing chamber of a mixing head. Here, however, the component mixture leaves the mixing chamber in an uneven or atomized manner, which has a disadvantageous effect on even and accurate pouring. Moreover, adding gas to the supply tank in order to mix it with the corresponding component has also proven to be extremely unreliable. When the gas is introduced into the component in the supply tank by means of an agitator, the gas again separates very quickly from the component, so that introducing the gas is largely ineffective.
In addition, DE-OS No. 25 44 559 also describes charging one or both components with gas before actual reaction mixing takes place. To do this, the stream of reaction component which is to be charged with gas is constricted and, by maintaining the respective component flow rate, a vacuum is produced in the axis of said constriction. The gas is added in the area of this vacuum and thereby mixed into the component. However, the size of the gas bubbles which can be achieved in the components using this method is not generally sufficient to prevent a large degree of bubble coalescence from taking place or to assure that the gas content will be highly dispersed in the components, in particular when such components must be kept on hand in tanks for a relatively long time.
Thus, the objective of the invention was to develop a process and device with which gas used in preparing a reaction mixture of at least two reaction components could be dispersed in one of the components in a simple manner in the most uniform and finely divided possible manner.